Test instrumentation is available for measuring the performance of various types of infrared (IR) detectors and preamplifiers under a variety of infrared photo backgrounds, gamma photon backgrounds, and neutron backgrounds at realistic detector operating parameters.
Where a broadband spectral IR source is utilized, it is necessary to employ a suitable filter or dispersive optics, however a single source will produce IR radiation over the entire spectral range. As dispersive optics are complicated and expensive, the use of filters has been prevalent in this specialized art.
It is considered a more desirable option to place the IR source externally, rather than internally, because the problem of calibration is reduced even though the problem of background radiation reduction in the dewar is increased, being the lesser of the two problems. However, background flux can be attenuated by placing a cold optical filter in the optical path especially at cryogenic temperatures, in the order of four degrees Kelvin. Past experience has shown that a dewar operating at liquid cryogen temperature, using liquid nitrogen and helium, can provide realistic operational temperatures for the apparatus.
Each filter, which is usually deposited on a germanium substrate, is supported on an aluminum member which can be a hub to enable it to be rotated in the IR application described above. Adequate thermal coupling of the liquid heat sink to the germanium substrates of the filters is provided so that the entire monochromator and the low-background chamber assembly will be isothermal. The coefficients of thermal expansion of the germanium and the aluminum hub differ by a factor of about three. Therefore, the interface member between the cooled aluminum hub and the germanium substrate must be able to relieve the stress arising from the difference in mechanical motion of these two materials while cooling. The interface member must also simultaneously maintain a thermal coupling between the substrate and the metal hub to allow sufficient short thermal response times. Thus, the interface member must have both thermal and stress-relieving qualities.
Heretofore, the interface member normally used consisted of a thin flat sheet of pure Indium Metal, about 0.010 inch thick. The Indium will flow under pressure to conform to the changing interface dimensions. However, with this arrangement a normal pressure of 50 p.s.i. must be maintained in order for the Indium interface member to be effective.
There are several disadvantages to the use of Indium as an interface member. The higher pressure that must be employed is a disadvantage from the standpoint of a higher probability of stress in the fragile germanium substrate, and requires a more complex constant pressure device. Furthermore, pure Indium is an expensive material. Both disadvantages are overcome by the present invention by using an interface member made of common household aluminum foil on which is impressed a waffled surface capable of resiliently supporting the fragile filter.